This invention relates to converting an oxygenate feedstock to an olefin product. In particular, this invention relates to converting an oxygenate feed to an olefin in a reaction apparatus in which catalyst is kept in a moving state throughout a reaction zone and a recirculation zone.
Demand for polyolefins, e.g., polyethylene and polypropylene, has been steadily increasing. It is projected that the increased demand for polyolefins will outpace the availability of raw materials, e.g., ethylene and propylene, from which polyolefins can be made.
Olefins which are used to make polyolefins have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking of the petroleum. The cost of petroleum cracking has steadily increased, however, making it important to find alternative feedstock sources for olefins.
Oxygenates are a promising alternative feedstock for making olefins. Particularly promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials such as coal, recycled plastics, municipal wastes, or any appropriate organic material. Because of the wide variety of sources, oxygenates have promise as an economical source for olefin production.
One way in which olefins can be made from the alternative oxygenate feedstocks is by catalytic conversion, hereinafter called an xe2x80x9coxygenate conversion reaction.xe2x80x9d In U.S. Pat. No. 4,499,327, for example, a catalytic process for converting methanol to olefins is described. The catalyst used in that process contains a silicoaluminophosphate (SAPO) molecular sieve.
Of course it is highly desirable to convert as much of the oxygenate feedstock as possible into as much ethylene and propylene as possible. U.S. Pat. No. 4,873,390 describes a method of increasing the amount of ethylene and propylene produced from the catalytic conversion of oxygenate feedstock, preferably in a fluidized bed reaction system, by controlling the amount of carbonaceous deposits on the catalyst returned from a step of contacting the catalyst with a regeneration medium to a step recontacting the regenerated catalyst with the oxygenate feedstock. The catalyst that is used in the process also contains a SAPO molecular sieve.
U.S. Pat. No. 6,023,005 also describes a method of increasing the amount of ethylene and propylene produced from the catalytic conversion of oxygenate feedstock, preferably in a fluidized bed reaction system, by controlling the amount of carbonaceous deposits on the catalyst returned from a step of contacting the catalyst with a regeneration medium to a step recontacting the regenerated catalyst with the oxygenate feedstock. The patent further discloses mixing the regenerated catalyst with portion of catalyst flowing out of the reaction zone and contacting the catalyst mixture with the oxygenate feedstock. The catalyst that is used in the process also contains a SAPO molecular sieve.
U.S. Pat. No. 6,166,282 discloses a method of reducing the amount of total catalyst required in the catalytic conversion of oxygenate feedstock and enhancing conversion to desired products, in a fluidized bed reaction system, by employing both a dense phase and a transition phase reaction zone, operating at distinct gas superficial velocities. Further, reference again is made to returning a portion of catalyst flowing out of the reaction zone to recontact with the oxygenate feedstock. Again, the catalyst used in the process contains a SAPO molecular sieve.
In view of the importance of how catalyst is managed in the reaction systems associated with conversion of oxygenate feedstocks to olefins over SAPO molecular sieves, improved processes are sought to obtain desired conversion products while inhibiting the conversion to undesirable byproducts. More specifically, improved fluidized bed oxygenate conversion processes are sought which provide optimal catalyst inventories within a reactor apparatus to enhance the conversion to the desired products and suppress the conversion to undesirable byproducts.
The present invention solves the current needs in the art by providing a method for converting an oxygenate feedstock to a product including ethylene and propylene in a fluidized bed reactor. One embodiment of the method of the present invention comprises the following steps: providing an oxygenate feedstock, a catalyst that incorporates a SAPO molecular sieve, and a reactor apparatus including at least a reaction zone and a recirculation zone, wherein the temperature in at least one point in each of the reaction zone and the recirculation zone is at least about 250xc2x0 C.; contacting the feedstock with the catalyst in the reaction zone under conditions effective to convert the feedstock to a product including prime olefins, the conditions including a GSV of at least about 0.1 m/s at at least one point in the reaction zone; having at least a portion of the catalyst in the reaction zone flow to the recirculation zone; and having a ratio of the mass of said catalyst in the reaction zone to that of the sum of the mass of the catalyst in both the reaction zone and the recirculation zone of between at least 0.01 and no greater than 0.99.
Another embodiment of the present invention is also directed to a method for converting an oxygenate feedstock to a product including ethylene and propylene in a fluidized bed reactor. The method comprises the following steps: providing an oxygenate feedstock, a catalyst that incorporates a SAPO molecular sieve, and a reactor apparatus including at least a reaction zone and a recirculation zone, wherein the temperature in at least one point in each of the reaction zone and the recirculation zone is at least about 250xc2x0 C.; contacting the feedstock with the catalyst in the reaction zone under conditions effective to convert the feedstock to a product including prime olefins, the conditions including a GSV of greater than about 0.5 m/s at at least one point in said reaction zone; recirculating the catalyst to establish a temperature differential; and having a ratio of the mass of said catalyst in the reaction zone to that of the sum of the mass of the catalyst in both the reaction zone and the recirculation zone of between at least 0.01 and no greater than 0.99.
Yet another embodiment of the present invention is directed to a method for converting an oxygenate feedstock to a product including ethylene and propylene in a fluidized bed reactor. The method comprises the following steps: providing an oxygenate feedstock, a catalyst that incorporates a SAPO molecular sieve, and a reactor apparatus including at least a reaction zone and a recirculation zone, wherein the temperature in at least one point in each of the reaction zone and the recirculation zone is at least about 250xc2x0 C.; contacting the feedstock with the catalyst in the reaction zone under conditions effective to convert the feedstock to a product including prime olefins, the conditions including a GSV of at least about 0.1 m/s at at least one point in said reaction zone; having an ACFE index in the reactor apparatus of at least about 1.0; and having a ratio of the mass of the catalyst in the reaction zone to that of the sum of the mass of the catalyst in both the reaction zone and the recirculation zone of between at least 0.01 and no greater than 0.99.
These and other advantages of the present invention shall become apparant from the following detailed description, the attached figures and the appended claims.